1. Field of the Invention
This invention relates to a semiconductor device, and more particularly to a semiconductor device wherein a cathode is divided into a plurality of discrete islands connected together through a plate and the cathode side thereof is pressed against the anode side thereof.
2. Description of the Prior Art
It is generally known in the field of high power semiconductor devices especially high power gate turn-off thyristors (GTO) that the cathode side of the element is pressed against the anode side thereof (pressure method). Conventionally, as is illustrated in FIG. 1, a structure in high power GTO's suitable for turning off large current is adapted such that a plurality of cathode emitters are formed in the form of islands, each of which is environed with gate electrode.
A semiconductor element 11 in FIG. 1 is a GTO and has a P-N-P-N four layer structure. A P type layer 11a represents an anode emitter region, an N type layer 11b an anode base region, a P type layer 11c a cathode base region and an N type region 11d a cathode emitter region. An anode electrode 10 is disposed on the anode emitter region 11a and is in electrical connection with the same. The anode electrode 10 generally comprises an alloy of a metal having a coefficient of thermal expansion hardly different from that of silicon (Si), for example, molybdenum (Mo). The cathode emitter region 11d includes many discrete islands on which cathode electrodes 12 are mounted so as to come into electrical contact with these islands. Disposed over the cathode base region 11c around the islands of the respective cathode emitter regions 11d are electrodes 13 which are held in electrical connection with the cathode base regions. The respective cathode electrodes 12 are electrically coupled to each other by intervention of a cathode insert 14 which is made up by a highly thermally and electrically conductive metal material such as molybdenum (Mo). In order that the respective gate electrodes 13 surrounding the cathode emitter regions 11d may not come into contact with the cathode insert 14 during the pressing of the element, the cathode base regions are dimpled by chemical etching or other well known methods. The cathode insert 14 is fixed to the GTO element 11 through the use of silicon rubber 18 or other conventional materials, no to move itself during assembly. Two electrode blocks 16 and 17 are disposed on the anode electrode 10 and the cathode insert 14 so as to come into electrical connection with the same, respectively. These electrode blocks 16 and 17 are made up by a material which is very high in thermal conductivity and has good affinity for the cathode insert 14 typically of copper (Cu), for example, molybdenum (Mo). By pressing the two electrodes blocks 16 and 17 to each other in the direction of the arrow, the GTO element with satisfactory heat radiation is attained.
As is well known in the art, the above described structure of the semiconductor device is suitable for high power applications because the anode and cathode electrode blocks 16 and 17 are highly effective in radiating heat. A particular problem with the above structure of the semiconductor device lies in the interfacial condition of the cathode insert 14 between the electrode block 17 and the cathode electrodes 12.
The cathode insert 14 is generally processed by etching, cutting, patching and otherwise. In this instance, a burr is necessarily brought at edge portions of the cathode insert 14. This burr is the main cause of short-circuit with the gate electrode during the pressing of the semiconductor device. One way to overcome the problem is to process the cathode insert 14 so as to remove such burrs. This approach however produces another difficulty from the viewpoint of process administration.